Structural materials which are exposed to aggressive environments are susceptible to cracking. This is particularly the case when the materials are subjected to various levels of cyclic or steady stress, and the resulting occurrence is often referred to as "stress corrosion cracking" or "corrosion fatigue". The nuclear industry has encountered this problem when structural materials (such as stainless steel piping) operate under sustained or cyclic stress in the presence of high temperature water. As described in U.S. Pat. No. 5,417,116 (Solomon et al.), damage from stress corrosion cracking is of greater concern than damage caused by uniform corrosion, since material failure in the first instance is far less predictable than in the second instance. Thus, methods for accurately and conveniently measuring the cracking phenomenon are of great importance.
One very useful technique for measuring crack growth in a corrosive environment involves the use of a double cantilever beam (DCB) sensor. These sensors usually include a pair of elongate beam portions opposed to each other, and a crack growth section formed in a connecting ligament between the beam portions, extending from an axially intermediate portion of each beam portion to a rear end thereof. As described in U.S. Pat. No. 5,378,429 (Hayashi et al.), a pre-cracking section is first formed in the starting end of the crack growth section. Crack growth is measured by a direct current potential method. Wedges inserted between the beam portions, or similar devices, are often used to maintain a selected stress intensity factor at the crack growth section. The Solomon and Hayashi patents provide illustrations of typical DCB sensors, as do U.S. Pat. No. 4,924,708 (Solomon et al.) and U.S. Pat. No. 4,677,855 (Coffin et al.). In the case of the nuclear industry, DCB sensors are often used in a boiling water reactor (BWR). The sensors are placed, for example, inside the pressure vessel of the reactor, in a position exposed to the flow of recirculation water.
A more specific description of the placement of the sensor should be preceded by a brief description of relevant components of a BWR. As those skilled in the art understand, BWR's and other types of reactors include a plurality of fuel rods grouped together to form a fuel assembly. A number of these assemblies are typically arranged in a matrix to form a reactor core. In BWR's, the fuel assemblies are usually grouped in clusters of four, with a control rod associated with each cluster. The control rods--provided with strong neutron-absorbing materials--serve to maintain control of the power level of the reactor. The control rods (or control "blades") in a BWR are often cruciform-shaped.
As described in U.S. Pat. No. 4,818,471 (Thomson et al.), strings of local power range monitors (LPRMs) are typically dispersed throughout the core of a BWR, between the corner locations of the fuel assemblies. Each string includes a hollow tube with multiple neutron detectors located at discrete axial locations. These detectors provide crucial local power monitoring information during operation of the reactor. Movable tip probes are used to calibrate the detectors at specific time intervals. The tip probes (sometimes referred to as "tip tube monitors") are usually inserted into selected detector string tubes from the bottom of the reactor core.
The cross-sectional size of a typical LPRM tube is usually not large enough to accommodate both a DCB sensor and a tip tube monitor. Thus, if a DCB sensor is inserted, a tip tube monitor usually cannot also be used in that tube. The loss of a site for the tip tube monitor would be a serious problem, because of the loss of local power monitoring information which would result from such a reactor design.
One possible way to overcome this problem is to reduce the size of the DCB sensor, which typically has a width of about 1.1-1.5 cm, and a height of about 2.5 cm-3.5 cm (dimensions "W" and "H" in FIG. 1, respectively). However, when the sensor is made smaller than those dimensions, the thickness of the ligament joining the beam portions must also be reduced. It thus becomes very difficult to manufacture and handle sensors with such thin ligaments. Even if a way is found to handle the small sensors, their fragile nature makes them susceptible to breakage. This in turn results in the inability to measure crack growth in a particular location, e.g., within a reactor vessel. Moreover, it is very costly to remove and replace sensors which break during operation of an apparatus like a BWR.
It should thus be apparent that new techniques for incorporating both a DCB sensor and a tip tube monitor into cavities like an LPRM tube would be welcome in the art. If these techniques involve reducing the size of the sensor, they must also involve maintaining its strength and durability. Moreover, a sensor having a reduced size must still fully perform its intended function, i.e., accurately measuring the crack growth of a metal in an aggressive environment. The sensor must also be fully compatible with any attached or surrounding components, and must be relatively easy to produce and incorporate into other equipment, such as an LPRM tube.